I've been reading about early modern military history and was struck by how small battles and armies were. In the Thirty Years' War, incredibly important battles such as Breitenfield and Lützen were fought between armies rarely numbering more than 35,000 men each. Whereas the Second Punic War saw numerous engagements between armies ranging from 40,000 to over 100,000 men.

Why were armies so much larger in the Punic War than they were during the Thirty Years War? Given what limited information I've seen on demographics it seems that the Romans and Carthaginians levied an enormous proportion of their populations during these conflicts, whereas the armies of the Thirty Years War were pretty small relative to the populations of the combatants. More generally, why were classical wars fought on a scale that wouldn't be seen again in the West until Napoleon?